title: Vacuum immobilisation reduces tumour excursion and minimises intrafraction error in a cohort study of stereotactic ablative body radiotherapy for pulmonary metastases.
abstract: BACKGROUND
  The purpose of this study is to assess the impact of a vacuum immobilisation system on reproducibility of patient set-up, interfraction stability and tumour motion amplitude.
  
  
  METHODS
  From February 2010 to February 2012 as part of a prospective clinical trial 12 patients with solitary pulmonary metastases had consecutive four-dimensional computed tomography (4DCT) scans performed with and without vacuum immobilisation. The displacement of the tumour centroid position was recorded in each of the 10 phases of the 4DCT reconstruction. A further six patients with seven metastases underwent single fraction stereotactic ablative body radiotherapy (SABR) during this period (a total of 19 targets) and were included in an analysis of positional reproducibility and intrafraction immobilisation. Couch shifts recorded in the medio-lateral (X), cranio-caudal (Y) and ventero-dorsal (Z) planes.
  
  
  RESULTS
  For the 19 treatments delivered, the median (0-90% range) shift required immediately pretreatment was 1 mm (0-3) in the X-plane, 2 mm (0-6) in the Y-plane and 4 mm (0-8) in the Z-plane, respectively. The mean (+/- standard deviation) of mid-treatment shifts were 0.3 mm (+/- 0.7), 1.1 mm (+/- 2) and 0.8 mm (+/- 1.5) in the X, Y and Z planes, respectively. Mid-treatment shifts were <2 mm in all directions (P = < 0.001). The length of treatment time correlated to the required shifts in the Z plane (r(2)  = 0.377, P = 0.005), but not in the X or Y planes (P = 0.198 and P = 0.653, respectively). In the subset of 12 patients who had two 4DCTs, the median (range) amplitude of tumour displacements in the X, Y and Z planes when immobilised were 0.9 mm (0.3-2.9), 2.6 mm (0.2-10.6) and 1.6 mm (0.5-5.5), respectively. Immobilisation reduced the volume of tumour displacement during respiration by a median of 52.6% (P = 0.021).
  
  
  CONCLUSIONS
  Vacuum immobilisation reduces total tumour excursion, facilitates reproducible positioning and provides robust intrafractional immobilisation during SABR treatments for pulmonary metastases.
authors.affiliation: Division of Cancer Imaging and Radiation Oncology, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia; Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Shankar
authors.initials: S
authors.lastname: Siva
authors.forename: Tomas
authors.initials: T
authors.lastname: Devereux
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
authors.forename: Suki
authors.initials: S
authors.lastname: Gill
authors.forename: Michael
authors.initials: M
authors.lastname: Macmanus
authors.forename: Mathias
authors.initials: M
authors.lastname: Bressel
authors.forename: Brent
authors.initials: B
authors.lastname: Chesson
authors.forename: Jason
authors.initials: J
authors.lastname: Callahan
authors.forename: Daniel
authors.initials: D
authors.lastname: Pham
authors.forename: Rodney
authors.initials: R
authors.lastname: Hicks
authors.forename: Farshad
authors.initials: F
authors.lastname: Foroudi
authors.forename: David
authors.initials: D
authors.lastname: Ball
mesh_terms: D000328:Adult; D000368:Aged; D015331:Cohort Studies; D019583:Dose Fractionation, Radiation; D005260:Female; D056973:Four-Dimensional Computed Tomography / Q000379:methods*; D006801:Humans; D007103:Immobilization / Q000379:methods*; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy / Q000556:secondary*; D008297:Male; D008875:Middle Aged; D056888:Patient Positioning / Q000379:methods*; D016634:Radiosurgery / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000379:methods; D016896:Treatment Outcome; D014618:Vacuum
keywords: immobilisation cradle; lung; motion; radiosurgery; stereotactic